A metabolite can be pharmacologically active without explaining a particular effect at a particular time. In a small rhesus-monkey experiment, plasma amphetamine appeared later than the strongest learned response to methamphetamine. The investigators therefore did not find support for a substantial contribution of the amphetamine metabolite to the discriminative cue they measured.
That conclusion concerns a trained behavioral endpoint under a specific dosing protocol. It does not establish that amphetamine is inactive, identify a human impairment threshold, or turn a methamphetamine-to-amphetamine ratio into a reliable clock.
What did “drug discrimination” mean?
Banks and colleagues studied three adult male rhesus monkeys weighing 6–11 kg. Two were initially experimentally naive; the third had earlier cocaine-discrimination experience, with no drug exposure for at least three months before this training began.
The monkeys learned to respond on one key after methamphetamine and another after saline. Food reinforcement depended on completing a fixed ratio of 30 responses. The training dose initially was 0.1 mg/kg intramuscular methamphetamine, but reliable discrimination was not established after 136 sessions. Investigators increased it to 0.18 mg/kg; criterion performance then took 63–73 sessions.
During tests, either key could produce food. The principal outcome was the percentage of responses on the methamphetamine-associated key, termed methamphetamine-appropriate responding. A value of 90% or more met the study’s definition of full substitution.
This percentage describes learned key selection. It is not a percentage of impairment, a verbal report of intoxication, or a measure of driving performance. Extensive training is part of the experimental context rather than evidence that untrained humans would show the same pattern.
Which drugs, doses and specimens were compared?
The experiments used (+)-methamphetamine hydrochloride and (+)-amphetamine hemisulfate, with doses expressed as their salts. Intramuscular doses of 0.032–0.32 mg/kg were tested. The 0.32 mg/kg dose of each drug was the lowest tested dose producing full substitution in all three monkeys and was selected for the concentration–effect comparisons.
Behavioral tests followed dosing at approximately 3, 10, 30, 56, 100, 180, 300 and 560 minutes. Neither drug significantly changed overall response rate under the tested conditions. Preserving that null finding matters: a change in which key an animal selected was not accompanied by a demonstrated general slowing or acceleration of responding.
Pharmacokinetic experiments used the same three monkeys but occurred on separate days from behavioral testing. Blood collected before dosing and at 3, 10, 30, 56, 100, 180 and 300 minutes was processed to plasma. The study therefore compared corresponding time courses across sessions; it did not measure every behavioral response alongside a simultaneous blood sample.
Did plasma methamphetamine and amphetamine peak together?
No. After 0.32 mg/kg intramuscular methamphetamine, individual peak plasma methamphetamine concentrations ranged from 38.7 to 67.5 ng/mL, with a group mean of 51 ± 8.5 ng/mL, reported as mean ± standard error. The observed peak occurred at 10 minutes in two monkeys and 56 minutes in the third.
The amphetamine metabolite remained below the assay’s detection limit until 56 minutes. Its observed peak occurred at 180 minutes in one monkey and 300 minutes in two. A result below detection is an analytical limit, not proof that absolutely no metabolite was present.
Direct administration of amphetamine was a separate condition. After 0.32 mg/kg intramuscular amphetamine, peak plasma amphetamine concentrations ranged from 31.3 to 42.9 ng/mL, averaging 37.2 ± 3.3 ng/mL. Those figures describe administered amphetamine, not the amount formed as a metabolite after methamphetamine. Confusing the conditions would materially change the interpretation.
What did the delayed metabolite imply about the learned cue?
Methamphetamine-appropriate responding was greatest early, when the amphetamine metabolite was undetectable or low. Later, when metabolite concentrations were highest, responding predominantly occurred on the saline-associated key.
The mismatch argues against a substantial role for the delayed amphetamine metabolite in maintaining this particular methamphetamine cue. Direct amphetamine administration nevertheless produced methamphetamine-appropriate responding. The experiment therefore distinguished the ability of administered amphetamine to substitute for the cue from the contribution of amphetamine formed after methamphetamine.
Other metabolites were not measured. The findings do not exclude every metabolite-related effect, effects under another exposure regimen, or effects on outcomes other than trained key selection.
Why did concentration and effect trace a loop?
When the investigators plotted learned responding against plasma concentration over time, the data formed clockwise hysteresis loops after either methamphetamine or directly administered amphetamine. At a similar plasma concentration, the measured cue could be stronger earlier and weaker later.
The authors interpreted this pattern as consistent with acute tolerance. It shows why a concentration–effect relationship may depend on where a measurement falls within the time course. It does not validate a numerical correction that can be applied to a person’s blood result.
With only three subjects, separate pharmacokinetic and behavioral sessions, and an extensively trained animal endpoint, the evidence cannot establish an individual human concentration–impairment curve. The reported concentrations are plasma values, not interchangeable whole-blood or urine values.
What can this study contribute to toxicology interpretation?
It offers a disciplined way to test a proposed explanation: identify the analyte, measure its timing, define the effect, and ask whether their time courses agree. Here, delayed metabolite formation did not track the strongest drug cue.
A forensic interpretation still needs its own specimen, collection time, analytical limits and functional evidence. Detection of methamphetamine or amphetamine alone does not establish impairment, and the disappearance of a learned animal cue does not establish that all pharmacological effects have ended.
Scientific source
Banks ML, Smith DA, Kisor DF, Poklis JL. Relationship between discriminative stimulus effects and plasma methamphetamine and amphetamine levels of intramuscular methamphetamine in male rhesus monkeys. Pharmacology Biochemistry and Behavior. 2016;141:58–65. doi:10.1016/j.pbb.2015.12.001. Full-text author manuscript. Historical original nonhuman-primate experiment.
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